GB2260412A

GB2260412A - Viscosity meter

Info

Publication number: GB2260412A
Application number: GB9221365A
Authority: GB
Inventors: Christopher Leigh-Jones; Steven Western
Original assignee: LEIGH JONES CHRISTOPHER
Current assignee: LEIGH JONES CHRISTOPHER
Priority date: 1991-10-11
Filing date: 1992-10-12
Publication date: 1993-04-14
Anticipated expiration: 2012-10-12
Also published as: GB9221365D0; GB2260412B

Abstract

In a method and apparatus for measuring the viscosity of a fluid using a modification of Stoke's law, the roll or fall of a ball 2 through the fluid along a tube 1 containing the fluid is timed and its viscosity calculated. The ball 2 is guided along the tube 1 by a guide means 3 located therein and arranged to provide at least two distinct spaced apart points of contact P with the ball 2 such that the ball 2 is substantially surrounded by the fluid as it moves along the tube 1. <IMAGE>

Description

Viscosity Meter The present invention relates to an apparatus and method for measuring the viscosity of a fluid.

Several devices are known for measuring the viscosity of fluids, in particular liquid hydrocarbon fuels and lubricants. The viscosity measurement is based on a modification of Stoke's Law which states that the steady velocity of a sphere falling vertically through a fluid of infinite volume is a function of the fluid's viscosity. For a practical application, this law must be modified to take account of the effect of the fluid film thickness around the sphere or ball as it falls through the fluid. This both guides the ball as it falls and restricts its terminal velocity.

A further modification of viscometers using the falling ball principle is to cause the ball to roll down an inclined tube with a quasi-rolling motion. The 'Flowers Viscometer' is a typical example of this type of device.

However, falling ball viscometers, whether vertical or inclined, suffer from a number of limitations.

Firstly, in such known devices the range of test fluid viscosities which can be accurately measured by a given ball diameter is limited. Typically, a device that will measure the viscosities of fluids ranging from diesel fuel (2cSt &commat; 40"C) to SAE 50 lubricating oil (19 cST &commat; Q 100 C) l00c) will require at least three different sized balls.

Further, turbulence of the fluid around the ball will cause the ball to oscillate and this can lead to significant errors. In order to overcome this, the spacing between the ball and the tube must be accurately controlled and kept very small ;n the case of vertical devices. In the case of inclined devices the angle of inclination of the tube should be kept very smalltypically less than 15 .

However, when the annular gap between the ball and the tube is very small, the device becomes very sensitive to thermal expansion of the tube and/or the ball. Tolerances, damage and temperature control of all of the components become very critical. Further, entrapped air will enter the annular gap providing a low viscosity reading by allowing the ball to accelerate momentarily. Also, if the tube inclination is very shallow this will restrict the range of viscosities that can be measured.

The object of the present invention is to provide a device which improves the accuracy, repeatability and simplicity of known viscosity meters.

According to one aspect of the present invention there is provided a viscosity meter comprising a tube for containing a fluid whose viscosity is to be measured; a ball which fits inside said tube and, in use, is caused to move along the length of the tube; and guide means, located in said tube and running substantially along the length thereof, to guide said ball; said guide means being arranged to provide in use at least two distinct spaced apart points of contact with the ball such that said ball is substantially surrounded by said fluid except at these points of contact.

Thus as the ball moves down the tube it need be in contact with, or in close proximity to, the tube walls only at the points of contact with the guides.

Relatively large spaces otherwise exist all round the ball through which the fluid can freely pass without any significant deterioration in accuracy due to thermal expansion etc. The use of guide means providing two points of contact with the ball also ensures that the ball is accurately guided. A greater angle of inclination can therefore be provided without ball oscillations occurring.

The form of the guide means may vary. For example, spaced apart parallel guide rails providing two points of contact with the ball in use may extend through the tube. Alternatively, the guide means may comprise two angled surfaces, preferably defining a general v-shape in cross-section, between which the ball runs and which engage the ball at distinct points at either side thereof at any time during its movement along the tube.

In such an arrangement, the guide surfaces may comprise the shaped walls of the tube itself, or may be provided by an elongate insert of v-shaped cross-section which is located within a type of tube of circular cross-section.

The latter arrangement is convenient to manufacture in that standard, cylindrical tubing may be used.

In order to increase the stability of the ball as it moves through the fluid along the tube the points of engagement of the ball with the guide means are preferably arranged such that the ball spins rather than rolls as it moves down the tube, i.e. it undergoes a greater number of revolutions about the horizontal axis perpendicular to the direction of movement that it would if rolling down a simple planar support surface for example. This occurs when the sideways spacing between the points of contact is significantly less than the diameter of the ball. We have found that the ball's stability is optimum when the spacing between the points of contact is around half the diameter of the ball.

Several available motion sensors may be used to detect the motion of the ball. In particular, sensors may be mounted towards either end of the guide means or the ball may be detected electronically using, for example, tuned coils, hall effect transducers or proximity probes mounted in the tube wall. Proximity probes are presently preferred.

Preferably the viscosity meter may be provided with means for automated temperature correction. A temperature probe, e.g. a platinum temperature sensor provided within the tube measures the fluid temperature which is processed using standard formulae programmed into the system's electronic circuitry. Thus the temperature reading can be used in the viscosity calculation to compensate for temperature variations of the fluid.

According to a second aspect of the invention there is provided a method of measuring the viscosity of a fluid using the viscosity meter of any preceding claim wherein said tube is substantially filled with said fluid; the ball is located on said guide means within said tube and caused to move through said fluid, along the length of said tube, on said guide means; the length of time taken for said ball to pass through a given length of said fluid is measured and the viscosity of said fluid is calculated based on said measured time.

A preferred embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings wherein: Fig. 1 shows a sectional view of a viscometer according to the present invention; Fig. 2 shows a perspective view of a viscometer tube according to the present invention; Fig. 3 shows a perspective view of a guide insert as used in the viscometer of the invention; Fig. 4 shows a preferred embodiment of a viscometer according to the invention.

The viscometer comprises a tube 1 containing a liquid 4 whose viscosity is to be measured, and a ball 2 which is allowed to fall through the liquid 4 along the length of the tube 1. The ball 2 is guided along the length of the tube 1 by a guide 3 such that the ball is substantially surrounded by the liquid 4 as it falls.

Sensors, preferably proximity sensors 5a, 5b, are located on the guide 3 to detect the motion of the ball 2 as it falls.

In use, in the preferred embodiment, the guide 3 is inserted in the tube 1 which is then filled with the liquid e.g. oil. The ball 2 is then placed in the tube 1 on the guide 3. End caps 6 on each end of the tube 1 enclose the liquid 4 therein. The tube 1 is tilted by a degree dependent on the type of liquid being measured and the ball 2 rolls along the guide 3. One proximity sensor 5a is located towards one end of the guide 3 and detects the motion of the ball 2 as it passes. A second proximity sensor 5b is located towards the other end of the guide 3 for detecting when the ball 2 approaches that end. Additional sensors may be provided along the guide 3 for improved accuracy. A platinum temperature sensor 7 may also be provided on the guide 3, e.g.

between the two motion sensors 5a, 5b, to provide for temperature effect corrections in the measurement.

In the preferred embodiment, the guide 3 consists of two angled metal planes in a V-shaped arrangement.

Alternatively the guide 3 may be a rhomboid or trapezoidal insert. Other shapes and arrangements of guide 3 may also be used. The guide 3 should be of such dimensions that as the ball 2 rolls along it, the ball 2 and guide 3 only touch each other at two distinct, spaced apart points P such that there is a gap between substantially the whole girth of the ball 2 and the tube 1 thus allowing the liquid to substantially surround the ball 2 except at the points of contact of the guide 3.

The fact that there are two distinct points of contact P between the ball 2 and the guide 3 make the ball tend to spin rather than roll. If, as is preferable, the spacing of the contact points P is less than the diameter of the ball 2, the ball 2 will tend to spin more and hence its stability against turbulence of the liquid is increased. For optimum stability the distance between the points P should be less than half the ball diameter.

In one embodiment, as shown in Fig. 4, the exterior of the tube 1 is shaped such that it can be tilted through an angle of arou9 30 up or down. This enables the same size ball 2 to be used for a wide range of viscosities.

The viscometer to which the present invention is most applicable is a small portable device, typically around 20 cm long and 3.5 cm in diameter.

The temperature sensor 7 allows for automated temperature correction using standard formulae programmed into the system's electronic circuitry (not shown). Using such means allows the viscometer to be used for a wide variety of fluids without needing to obtain a steady temperature before testing. This method relies on the assumption that the viscosity-temperature relationship of the test fluid is known or can be reliably estimated. For liquid hydrocarbon fuels and lubricants a typical operating range would be 0-100"C, preferably 10-60 C. Small changes in the viscosity index (VI) of lubricants have very little effect over this range.

The apparatus can be used for most of the liquid hydrocarbon lubricants and distillate fuels commonly used in the diesel engine, power or gas turbine industries. Residual fuel oils may also be measured provided the oil is first heated and all tests are conducted while the oil remains above its cloud point.

A wide range of hydrocarbon fluids have been tested, for example: DIESO F-76 1.7-4.3 cST &commat; 40"C (diesel fuel) OMD 113 11.9-13.5 cST &commat; 100 C (diesel lubricant) AVCAT/FS11 8.8 cST &commat; -20 C (turbine fuel) OX-38 39 cST &commat; Q 38 C 380C (turbine lubricant) Repeatabil is within + 5%, more typically + 1%, over a wide range of steady operating temperatures.

Improved repeatability and a device independent of orientation would be obtained by using two parallel viscometers of the type described, one of which contains a fluid of known and stable viscosity as a reference for the other.

Claims

1. A viscosity meter comprising a tube for containing a fluid whose viscosity is to be measured; a ball which fits inside said tube and, in use, is caused to move along the length of the tube; and guide means, located in said tube and running substantially along the length thereof, to guide said ball; said guide means being arranged to provide in use at least two distinct spaced apart points of contact with the ball such that said ball is substantially surrounded by said fluid except at these points of contact.

2. The viscosity meter of claim 1 wherein said ball is in contact with said guide means at just two distinct, spaced apart points of contact.

3. The viscosity meter of claim 1 or 2 wherein said guide means comprises two angled guide surfaces.

4. The viscosity meter of claim 3 wherein said guide means comprises a generally v-shaped insert for location in a generally cylindrical tube.

5. The viscosity meter of any preceding claim wherein said guide means is shaped such that the spacing between said points of contact is significantly less than the diameter of said ball such that in use the ball spins about a horizontal axis perpendicular to its direction of movement.

6. The viscosity meter of claim 5 wherein said spacing between the points of contact is around half the diameter of said ball.

7. The viscosity meter of any preceding claim further comprising sensing means for sensing the motion of said ball along said tube.

8. The viscosity meter of claim 7 wherein said sensing means comprises one or more motion sensors located on said guide means.

9. The viscosity meter of claim 7 wherein said sensing means comprises one or more motion sensors located on the wall of said tube.

10. The viscosity meter of claim 7, 8 or 9 wherein said sensing means comprises one or more proximity sensors.

11. The viscosity meter of claim 7, 8 or 9 wherein said sensing means comprises one or more electromagnetic sensors.

12. The viscosity'meter of any preceding claim further comprising temperature sensing means.

13. The viscosity meter of claim 12 wherein said temperature sensing means is located on said guide means.

14. The viscosity meter of claim 12 or 13 wherein said temperature sensing means comprises a platinum temperature sensor.

15. The viscosity meter of any preceding claim further comprising end caps removably located at either end of said tube.

16. The viscosity meter of any preceding claim wherein said tube is externally shaped to allow said tube to be tilted about a pivot point.

17. The viscosity meter of claim 16 wherein said tube can be tilted about said pivot point by 30 in each direction.

18. A method of measuring the viscosity of a fluid using the viscosity meter of any preceding claim wherein said tube is substantially filled with said fluid; the ball is located on said guide means within said tube and caused to move through said fluid, along the length of said tube, on said guide means; the length of time taken for said ball to pass through a given length of said fluid is measured and the viscosity of said fluid is calculated based on said measured time.

19. The method of claim 18 wherein said tube is inclined.

20. The method of claim 18 or 19 wherein the temperature of said fluid is measured and used to automatically correct said viscosity calculation to a reference temperature.

21. A viscosity meter substantially as hereinbefore described with reference to the accompany drawings.

22. A method substantially as hereinbefore described with reference to the accompany drawings.